How's this for a contradiction? On stage, the musical Once, now in previews at Broadway's Bernard B. Jacobs Theatre, is remarkably elegant. Based on the 2008 indie movie, it tells the bittersweet tale of two musicians who meet in Dublin, make folk music, and fall in love. Just like their songs, the production ripples with emotion. In one scene, a character sings for a banker to convince him he deserves a loan. The song is so devastating that the other bankers, sitting at their desks, lift their arms in unison. The quiet gesture tells us exactly how art has united them.

Yet for all its careful beauty, Once is also the product of coincidence. Barely a year ago, the creative team had a workshop at American Repertory Theater in Cambridge, Massachusetts. As they developed the show, they were inspired by what happened to be around them.

"We were all like college kids working together for four weeks," says Bob Crowley, who designed the set and costumes. "We did this thing in a tiny little damp church hall basement, and the aesthetic evolved from the rehearsals."

Take the set: Though scenes unfold in bedrooms, cafes, and recording studios, Crowley places the entire production in a pub. When they're not performing, the actors sit on the side of the stage, holding instruments they frequently play. If you squint your eyes, you can almost imagine a rehearsal room. "There was a line of tables that they sat on that just happened to be in the room, and with the whole cast playing their own instruments, those instruments just lined up at the side of the rehearsal room," Crowley says. "That little ground plan evolved very organically and hasn't changed."

When the workshop ended, it had a few performances on the set of a play that was already running at A.R.T. That play was staged in a roller disco, which meant there was a giant mirror on the back wall. Crowley liked seeing actors' reflections, so when he expanded his own set for Once's Off Broadway debut at New York Theatre Workshop, he covered it with mirrors.

The costumes followed the same path. Crowley recalls, "I went around with the head of wardrobe, and we went through their stock at A.R.T. And what I couldn't find there, I then went out vintage shopping all around Boston with her." Many of those original costumes, which evoke shabby artists and working-class immigrants, are still in the show.

Crowley values the spirit of those early designs. Discussing director John Tiffany and choreographer Steven Hoggett, he says, "From what they were doing in the room, it was obvious to me that from that, what would emerge would be a quiet aesthetic. I thought, 'Well, I've got to somehow encase this and embrace it.' And protect it, actually, is the word. Protect the show and the story and the fragility of it."

But his designs don't just protect the show's tone. They enhance it. The mirrors, for instance, tell us Crowley's pub is slightly magical. "They fracture the space, they fracture events," he says. "If you look in the mirrors, you can see part of a face or part of hand playing a guitar. [Once] is a modern fairy tale in lots of ways. It's odd and eccentric and told at a bit of an angle, and that's what those mirrors are part of."